JavaScript RegExp不起作用

Question

Why these return null :

var str="Is this all abc bbb c is";
var patt1=/is(?=bbb)/;

var str="Is this all abc bbb c is";
var patt1=/is(?=bbb)/;

var str="Is this all there is";
var patt1=/is(?=all)/; // <------ (?=all) vs (? =all)

but this returns is :

var str="Is this all there is";
var patt1=/is(?= all)/;

?

Answer 1

(?=...)

is the regex "look-ahead" function, so here is what your regular expressions mean:

/is(?=bbb)/ //look for isbbb and return is
/is(?=all)/ //look for isall and return is

Both of those do not exist in your string, so you get no matches.

Answer 2

Let's take your regular expression /is(?=bbb)/ . Imagine a little caret at the beginning of your expression (a caret is that blinking line between letters which shows where your text cursor is - it's behind the letter it's pointing to):

is(?=bbb)
^

We look for a small i in your string. We find one in the word this . The caret shifts:

is(?=bbb)
 ^

Is the next letter an s ? It sure is. The caret shift again:

is(?=bbb)
  ^

Now something interesting happens. Now we check if the next letter is b . However even if we find it we don't advance the caret.

If we match the subexpression bbb only then does the caret advance to the character after the matching parenthesis (however the text matched by the subexpression isn't included in the matched string):

id(?=bbb)
         ^

Unfortunately no where in your string is is being followed by bbb . That's why you're getting null .